1. Field of the Invention
One aspect of the present invention relates to a method for treating a substrate prior to bonding. Another aspect of the present invention relates to a method for bonding substrates.
2. Background Art
In the automotive industry, substantial resources are expended in developing effective windshield bonding processes to account for the typical contamination present under manufacturing conditions. Once a windshield is formed and before the windshield is bonded to the vehicle frame, contaminants may collect and/or settle on the windshield, thereby dirtying the windshield. The existence of contaminants on windshields was documented as an “Industry Update” in the March/April 2003 issue of AutoGlass magazine, available at http://www.glass.org/autoglass/maraprnews.htm. This article proposes the application of a solvent cleaning solution followed by manual scrubbing for cleaning and prepping the dirty windshield before bonding.
According to another proposal, the process of bonding automotive glass, e.g. windshields, side windows, and rear windows, includes the duplicate application of a primer for cleaning and priming the glass substrate, followed by a second primer step, followed by adhesively joining the glass substrate to a vehicle frame with an adhesive.
The first priming step includes priming the glass substrate with a red primer, for example, Betaseal 43519, available from Dow Chemical of Midland, Mich. The red primer contains a coupling agent to chemically modify the substrate surface. The red primer is highly diluted in a solvent blend that is similar to that typically used for other generic cleaning applications carried out at an automotive manufacturing facility, e.g. the cleaning of body surfaces before painting and the cleaning of body side moldings before adhesive bonding. Non-limiting examples of suitable solvents include any of a variety of aprotic solvents, such as toluene and xylene.
The second priming step includes priming the glass substrate with a black primer, for example Betaseal 43520A, available from Dow Chemical. The black primer is typically a thicker paint-like layer that chemically bonds to the coupling agent imparted by the first red primer layer, and adds bonding functionality to adhere to the subsequently applied adhesive material.
Problematically, the red and black primers cannot be consistently and reliably applied in an assembly plant environment. In general, both primers are commonly applied by hand, which may involve human error. For instance, in automotive manufacturing facilities, the red primer is commonly referred to by name as a “cleaner”. If the red primer is mistakenly identified as simply a cleaning solvent, the red primer may be applied as if it were a cleaning solvent. Since a solvent cleaning step does not require as much precision as a primer priming step, this mistake may detrimentally affect the effectiveness of the red primer application. If the glass is perceived already as a clean surface, the step may be mistakenly viewed as unnecessary, instead of being viewed properly as applying a coupling agent for the black primer. This mistake may lead to the omission of the red primer application step.
Moreover, the red primer is invisible to the human eye once applied to the glass surface. Yet, the black primer must be applied exactly over the area of application of the “invisible” red primer. If the application does not match, the resulting adhesive bond between the glass and the frame may be compromised. Further, the black primer only adheres to a glass substrate that has been modified by the coupling agent of the red primer, and does not directly adhere to a fritted glass. Thus, if the black primer is not applied correctly, adhesion to the fritted glass may not occur.
If the adhesion is unacceptable, the windshield may be scrapped, or used for other purposes, at a cost to the manufacturer. Additionally, environmental concerns exist regarding the solvent cleaner, and the red and black primers, as these compounds may contain volatile components that result in the necessity of venting solvent fumes.
In light of the foregoing, a method of treating substrates for bonding that increases reliability of the resulting substrate bond is needed. What is also needed is a method that simplifies and/or automates the bonding process.